Different asphaltenes containing hydrocarbon streams have different precipitating and fouling characteristics with regard to heated oil refinery surfaces. The problem of predicting the offending substances in a particular stream such as crude oil which foul heat-exchanger equipment in oil refineries and petrochemical plants has been virtually unresolved. Equipment fouling by heated hydrocarbon streams which results in carbonaceous deposits on heat-exchanger surfaces leads to a blockage of flow and a decrease in heat transfer. Both resulting conditions severely reduce heat efficiency in the processing of the crude oil. If it can be predicted which crude oils are troublesome, measures can be taken in advance to prevent this fouling by either removing the offending substances causing the deleterious deposits, or by adding antifouling additives to the flow stream to reduce depost formation. Therefore, it would be most desirable to be able to predict these streams with fouling tendencies.
There are a number of methods available for determining the rates of fouling of hydrocarbon streams. Conceptually, they are all similar in that they attempt to measure the change in heat transfer from a heated surface to a test fluid.
One approach is to use a test unit which is configured to allow measurement of the fluid temperature at the exit of the heat-exchanger while the metal temperature of the heated tube is controlled. This configuration provides for close simulation of refinery and petrochemical plant heat-exchanger operations and provides for measurement of the significant effect of fouling which is indicated by the reduction of heat transfer. The test unit provides for a thermal fouling evaluation of the crude oil in an accelerated test which is designed to reproduce the fouling problem experienced in a refinery over several months. Acceleration is provided by carrying out test operating temperatures higher than those in a particular refinery unit, so that the prospective level of fouling can be produced in a reasonable period of time (usually 3-4 hours). Heat transfer data is obtained by holding the heater tube at a constant temperature, while measuring the change in the liquid outlet temperature. As fouling progresses, i.e., a carbonaceous deposit build up on the heater tube surface, a decrease in the fluid outlet temperature results when using a constant outlet liquid temperature operation. The change in liquid outlet temperature with time provides the basic heat data required for comparative evaluation of untreated material and additive-treated material. The rate of change in outlet liquid temperature versus time shows relative fouling tendencies.
Present test equipment is only capable of measuring the overall tendency of heated hydrocarbon streams to foul refinery equipment and cannot predict which are the offending substances or fractions.
It is an object of this invention to provide a method which will predict the fouling tendency of asphaltene containing hydrocarbon streams such as residual and crude oils. These and other objects of the invention will be apparent from the following text.